This invention relates to the field of data communications, and in particular, circuits for transmitting and receiving a data signal on a local loop.
Digital subscriber line (DSL) technology is often used to facilitate data communications at relatively high data rates over a local loop between a telephone central office and a customer premise. In a typical configuration, a DSL modem is placed at the customer premises and communicates with a corresponding DSL modem at the central office. From the central office, the data signal is routed to a destination DSL modem through a digital network.
It is often the case that the local loop, which generally comprises a two wire pair, extends up to 18,000 feet from the central office to the customer premises. It is desirable that the DSL modems operate reliably over such long local loops.
To address this concern, the central office will often transmit a data signal to the customer premises of relatively high peak-to-peak voltage to ensure that the DSL modem at the customer premises receives a signal of adequate strength for demodulation to recreate the data signal transmitted. In fact, a typical central office line drive amplifier is designed to transmit a signal of up to 35 volts peak-to-peak across the local loop to the DSL modem at the customer premises.
Generally, this signal is attenuated significantly in local loops of significant length before reaching the DSL modem at the customer premises. Accordingly, the typical receive circuitry of DSL modems used at the customer premises is designed with the assumption that the data signal transmitted by the central office will be attenuated over the local loop. However, where a local loop is short in length due to the relatively close proximity of the customer premises to a central office, the 35 volt peak-to-peak signal is often clipped in the receive circuitry of the DSL modem located at the customer premises, as the receive circuitry was designed for lower voltage levels due to the attenuation in the local loop.
One solution to the unwanted clipping problem involves attenuating the central office data signal at the central office. However, the central office data signal may be attenuated generally by no more than 6dB due to cross talk noise considerations which leaves a peak-to-peak voltage of 17.5 volts. In the case where the local loop presents no significant attenuation of the central office data signal, a peak-to-peak voltage of 17.5 volts still causes clipping in the receive circuitry of the DSL modem located at the customer premises. Transmission of a data signal from the central office at a lower voltage to avoid the clipping problem is unfeasible as cross talk may cause too high a signal to noise ratio (SNR) for proper operation.
A second solution is to introduce attenuation circuitry between the local loop and the receive circuitry in the DSL modem that performs an attenuation function. This attenuation circuitry is best placed in the DSL modem at the local loop connection. Generally, this attenuation circuitry achieves the attenuation function by switching resistors in series with the amplifier input resistors with a complementary metal-oxide semiconductor (CMOS) analog switch. Unfortunately, one problem with this approach is that CMOS switches have increased xe2x80x9conxe2x80x9d state resistance when powered at low voltage, such as the +5 volt single rail power supply usually used in the transceiver of a customer premises DSL modem. This xe2x80x9conxe2x80x9d state resistance degrades performance in the unattenuated mode of the receiver. CMOS switches also present further disadvantages in that they are costly and require a circuit of greater physical size.
Consequently, there is a need for a transceiver circuit in a customer premises DSL modem to provide attenuation of a data signal transmitted from a central office over a local loop of minimal length, without additional cost and circuit size. Also, it is desirable that such a circuit avoid the introduction of unwanted resistance, etc.
The present invention is an economical transceiver circuit and method that for the attenuation of a downstream data signal transmitted from either a central office or a customer premise over a local loop of minimal length. The present invention may also be employed where the local loop is of sufficient length that additional attenuation is not needed.
Accordingly, the transceiver circuit of the present invention is placed in a modem and is comprised of a line drive transformer, a transmission circuit coupled to the line drive transformer, and a receive circuit coupled to the line drive transformer. The transmission circuit and the receive circuit are coupled to a primary winding of the line drive transformer and a secondary winding of the line drive transformer is coupled to a local loop at the central office or the customer premises. The receive circuit features an amplified signal pathway defined by a summing circuit having a receive operational amplifier coupled to a first input resistance and a feedback resistance. The amplified signal pathway is employed when the downstream data signal does not require attenuation (e.g. where the local loop is of significant length). In such cases, the amplified signal pathway amplifies the downstream data signal for further processing by the modem. This is accomplished by placing the receive operational amplifier is an active state. The receive circuit also features an attenuated signal pathway defined by the first input resistance in series with the feedback resistance. The series resistance is instituted by placing the receive operational amplifier in an inactive state. The series resistance attenuates the downstream data signal input for further processing by the modem.
In accordance with one aspect of the present invention, a method is provided for receiving a downstream data signal in a transceiver circuit of a modem. The method includes the steps of coupling a transmit circuit to primary winding in a line drive transformer, coupling a receive circuit to the primary winding, and coupling a secondary winding of the line drive transformer to a local loop. Next, the method includes the step of amplifying an downstream data signal with an amplifier having a first input resistance and a feedback resistance in the receive circuit, when the voltage level of the downstream data signal is below a predetermined threshold. The final step involves attenuating the downstream data signal with a series resistance comprising the first input resistance and the feedback resistance, the amplifier being disabled when the voltage level is greater than or equal to the predetermined threshold.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.